Production of chromium compounds



Patented Nov. 4, 1947 UNITED STATES PATENT OFFICE No Drawing.Application January 26, 1942, Serial No. 428,207

7 Claims.

1 This invention relates to chromium recovery and has for an object theprovision of an improved method or process for recovering chromium inthe form of relatively pure compounds or concentrates from crudechromiumbearing material such as chromite ore. More particularly, theinvention contemplates the provision of an improved method or processfor producing chromium oxide-bearing products. A further object of theinvention is to provide certain improvements in methods or processes forproducing chromic oxide as such or in chemical combination with a basicoxide such as calcium oxide. The invention also contemplates theprovision of an improved method or process for producing calciumchromate. A specific object of the invention is to provide an improvedmethod or process for utilizing alkali metal compounds in obtainingrelatively pure chromium compounds from crude chromium-bearing materialssuch as chromite ore.

Throughout the specification and in the claims, the term chromite ore isintended to include natural chromite ore, chromite ore concentrates,altered chromite ore and similar chromite mineral-bearing materials.

According to some heretofore customary proc-. esses, chromite ores aresubjected to oxidizin or roasting treatments in the presence of one ormore alkali metal compounds such, for example, as sodium carbonate,sodium hydroxide and sodium sulphate under such conditions as to oxidizethe trivalent chromium of the chromite ore to the hexavalent state andproduce alkali metal chromates such, for example, as sodium chromate.Sodium chromate thus produced is separated from the product of theroasting or dependently of the chromate production operations and,consequently, the sodium oxide content of the chromate compound iswasted or, at any rate, is not recovered in a form in which it isreadily available for re-use in the chromate production operations.Therefore, chromic oxide,

calcium chromite and calcium chromate are relatively costly commercialproducts. Such compounds are highly desirable materials for manyindustrial uses, but their usefulness is not fully developed because ofhigh costs.

The present invention provides for the production of such compoundsunder conditions and according to procedures such that the alkali metaloxide content of chromates employed in their production is utilized forthe production of additional chromate. A complete preferred process ofthe invention comprises (1) oxidation or roasting in air of chromite orein the presence of one or more alkali metal compounds in accordance withwell known oxidizing or roasting procedures with the production ofalkali metal chromate, (2) separation of the chromate from other mate-.rials associated therewith in the oxidized or roasted charge as, forexample, by leaching with an aqueous liquid followed by evaporation andcrystallization, (3) reduction of the chromate directly or afterconversion to dichromate with the production of chromic oxide as such orchromic oxide chemically combined with a basic oxide and one or morealkali metal compounds suitable for use in the oxidation or roastingtreatment of chromite ore, and (4) utilization of the one or more alkalimetal compounds thus pro duced in an oxidizing or roasting treatment ofadditional chromite ore. Through the recovery and re-use of the alkalimetal oxide of the chromate, the costs of producing compounds such aschromic oxide, calcium chromite and calcium chromate are reducedmateriallly and a wider field of usefulness for such compounds isprovided.

The invention also provides improved, more efiicient and less expensivemethods or processes for utilizing commercial alkali metal chromates anddichromates in the production of compounds such as chromic oxide,calcium chromite and calcium chromate.

According to the invention, an alkali metal chromate, such as sodiumchromate (Na2CrO4) or sodium dichromate (Na2Cr2O7), in finely dividedform, is intimately mixed with finely divided solid carbonaceous reducinmaterial and ignited. Finely divided charcoal, coke or coal may be em!ployed satisfactorily as the reducting material. Finely divided charcoalmay be employed most advantageously for the production of pure products.Finely divided coke or coal may be employed when relatively impureproducts are satise factory. When coal or coke is employed, the productsresulting from reduction of the chromium of the chromate throughelimination of a portion of the oxygen may be contaminated with coal orcoke ash.

My investigations have indicated that reduction of alkali metal chromateand dichromate with carbon through ignition of intimate mixtures of thematerials proceeds somewhat according to the following equations:

It will be noted that the above equations indicate the production ofsodium chromite (NazO-CrzOs) and carbon monoxide or carbon dioxide inreducing sodium dichromate with carbon in different proportionsrelatively to sodium dichromate and the production of sodium chromite(NazO -CI2O3), sodium carbonate and carbon monoxide in reducing sodiumchromate. When the solid reaction product resulting from the reductionof sodium dichromate with carbon is digested with water (hot or cold) asolution of sodium hydroxide substantially free of sodium carbonate anda solid residue consisting essentially of chromic hydroxide (Cr(OII)s)are produced, When the solid reaction product resulting from reductionof sodium chromate with carbon is digested with water, a solution ofsodium hydroxide and sodium carbonate and a solid residue consistingessentially of chromic hydroxide (Cr(OH)s) are produced.

Digestion of the solid products resulting from reduction does not effecthydrolysis or decomposition of all of the sodium chromite (NazO-Cr203)contained therein, and the solid residue consisting essentially ofchromic hydroxide contains some sodium oxide probably chemicallycombined with chromic oxide as sodium chromite. The residual sodiumoxide may be separated from the bulk of the chromic hydroxide bysubjecting the residue resulting from digestion to an oxidizingtreatment with air at an elevated temperature (above about 500 C.) andleaching the resulting product with water. The residual sodium oxide isconverted to sodium chromate in the oxidizing treatment and the chromichydroxide is Cn-' verted to chromic oxide (CI203) Calcium chromite andsodium carbonate may be produced by forming and igniting an intimatemixture of finely divided sodium chromate or sodium dichromate, finelydivided calcium oxide and finely divided carbon. It appears that thereactions proceed somewhat according to the following equations:

The sodium carbonate and calcium chromite produced in accordance withthe reactions indicated by the above equations may be separated bydigesting the solid reaction products with water, hot or cold. Calciumoxide may be employed as such or in chemical combination with carbondioxide as calcium carbonate.

Equations 5 to 7 indicate the production of a compound approximating inchemical composition di-calcium chromite. Compounds approximating inchemical composition mono-calcium chromite or tri-calcium chromite maybe produced by adjusting the proportions of chromate or di-chromate andcalcium oxide employed.

All of the reactions indicated by Equations 1 to 6 above are exothermicand usually proceed to completion without the application of heat fromextraneous sources. When the calcium oxide is employed in the formofcalcium carbonate and when the production of tri-calcium chromite issought, it may be advisable to provide additional heat from extraneoussources to insure eifective chemical combination of the calcium oxidewith the chromic oxide.

Calcium chromate may be produced by roasting in air a solid residueobtained in the digestion of a solid reaction product resulting from thereduction of a chromate or dichromate in the presence of calcium oxideto produce calcium chromite, Substantially complete conversion tocalcium chromate may be accomplished by roasting the di-calcium chromiteand the tri-calcium chromite. Roasting of the mono-calcium chromite willresult in the production of calcium chromate and chromic oxide. Completeconversion to chromate of the mono-calcium chromite may be accomplishedby incorporating in the roasting or oxidizing charge at least onemolecule of calcium oxide for each molecule of calcium chromitecontained therein.

In a complete preferred process of the invention involving the oxidationof chromite ore to produce sodium chromate, sodium carbonate, sodiumhydroxide or mixtures of sodium carbonate and sodium hydroxide obtainedby digesting the solid products resulting from reduction of sodiumchromate and sodium dichromate may be employed in the oxidationtreatment. The solutions obtained by digestion of the products ofreduction may be evaporated partially or entirely to obtain solid sodiumhydroxide or sodium carbonate or both, or to obtain sodium hydroxide, orsodium carbonate or both partly in the solid state and partly insolution for re-use in the oxidation of chromite ore. Solutionscontaining sodium hydroxide may be passed in contact with gases, such asflue gases, containing carbon dioxide to carbonate the solution, orpartly for the purpose of carbonating the solution and partly for thepurpose of washing the gases, and produce, and convert the sodiumhydroxide to, sodium carbonate or sodium bicarbonate or both.

The production of chromic oxide by reduction of sodium chromate withcarbon is illustrated by the following example:

A reaction mixture was formed by grinding together 648 pounds of sodiumchromate (Na2CI'O4) and 46 pounds of carbon in the form of charcoal. Themixture was placed in a crucible, covered with finely divided charcoaland ignited by means of a gas flame. Upon ignition, the reactionproceeded quietly and smoothly.

The amount of carbon employed represents an excess of about thirtypercent (30%) over that required to effect reduction of the chromate. Itis desirable that suflicient excess carbon be employed to preventre-oxidation of the reaction product during cooling.

The solid reaction product was cooled under nonoxidizing conditions,digested by grinding in water to form a pulp to eifect solution ofsodium compounds and filtered. Hot or cold water may be employed fordigestion. The filtrate obtained was an aqueous solution of sodiumhydroxide and sodium carbonate. The solid residue consisting essentiallyof chromic hydroxide (Cr(OI-I)3) was roasted in air at an elevatedtemperature (1000 C.) to eliminate water and carbon and convert thechromic hydroxide to chromic oxide. The roasted product was 95 percentchromic oxide.

The chromic oxide-bearing product was leached with water to removesoluble compounds such as sodium chromate contained therein. The residueremaining after leaching was substantially pure chromic oxide. Thesodium chromate thus obtained may be returned to the process. Conversionto chromic oxide of 97.5 percent of the chromium of the sodium chromatewas effected.

I claim:

1. The method of recovering chromium values from chromite ore whichcomprises subjecting the ore to an oxidizing treatment in the presenceof sodium carbonate to produce sodium chromate, treating the product ofthe oxidizing treatment to separate sodium chromate from other materialassociated therewith, subjecting the sodium chromate to a reducingtreatment with a carbonaceous reducing agent in the presence of calciumoxide and forming a product comprising sodium carbonate and calciumchromite, separating the sodium carbonate from the calcium chromite, andutilizing the sodium carbonate in an oxidizing treatment of additionalore.

2. The method of producing calcium chromite which comprises subjectingsodium chromate to a reducing treatment with a carbonaceous reducingagent in the presence of calcium oxide to eliminate a portion of theoxygen of the chromate and form a product comprising calcium chromiteand sodium carbonate, and separating the calcium chromite and the sodiumcarbonate.

8. The method of producing calcium chromate which comprises subjectingsodium chromate to a reducing treatment with a carbonaceous reducingagent in the presence of calcium oxide to eliminate a portion of theoxygen of the chromate and form a product comprising calcium chromiteand sodium carbonate, separating the calcium chromite and the sodiumcarbonate, and subjecting the calcium chromite to an oxidizing treatmentto oxidize chromium contained therein to the hexavalent condition.

4. The method of producing calcium chromate which comprises subjectingsodium chromate to a reducing treatment with a carbonaceous reducingagent in the presence of calcium oxide to eliminate a portion of theoxygen of the chromate and form a product comprising calcium chromiteand sodium carbonate, calcium oxide being employed in amount such thatthe calcium chromite produced contains more than one molecule of calciumoxide for each molecule of chromic oxide contained therein, separatingthe calcium chromite and the sodium carbonate, and subjecting thecalcium chromite to an oxidizing treatment to oxidize chromium containedtherein to the hexavalent condition.

5. The method of producing calcium chromite which comprises subjectingsodium chromate to a reducing treatment at an elevated temperature withsolid carbon in the presence of calcium oxide to eliminate a portion ofthe oxygen of the chromate and form a product comprising calciumchromite and sodium carbonate, and separating the calcium chromite andthe sodium carbonate.

6. The method of producing calcium chromate which comprises subjectingsodium chromate to a reducing treatment at an elevated temperature withsolid carbon in the presence of calcium oxide to eliminate a portion ofthe oxygen of the chromate and form a product comprising calciumchromite and sodium carbonate, separatin the calcium chromite and thesodium carbonate, and subjecting the calcium chromite to an oxidizingtreatment to oxidize chromium contained therein to the hexavalentcondition.

7. The method of producing calcium chromate which comprises subjectingsodium chromate to a reducing treatment at an elevated temperature withsolid carbon in the presenc of calcium oxide to eliminate a portion ofthe oxygen of the chromate and form a product comprising calciumchromite and sodium carbonate, calcium oxide being employed in amountsuch that the calcium chromite produced contains more than one moleculeof calcium oxide for each molecule of chromic oxide contained therein,separating. the calcium chromite and the sodium carbonate, andsubjecting the calcium chromite to an oxidizing treatment to oxidizechromium contained therein to the hexavalent condition.

MARVIN J. UDY.

REFERENCES GITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name I Date 1,526,325 Drefahl Feb. 17, 19251,955,326 .Demant Apr. 17, 1934 1,975,338 Vetter Oct. 2, 1934 2,098,176Udy Nov, 2, 1937 2,199,929 Van Wirt May 7, 1940 2,229,495 Udy Aug. 11,1942 1,924,710 Demant Aug. 29, 1933 FOREIGN PATENTS Number Country Date509 Great Britain 1- Feb. 17, 1872 5,948 Great Britain Mar. 13, 1884OTHER REFERENCES Mellor, Comprehensive Treatise on Inorganic andTheoretical Chemistry, 1931, vol. 11, pp. 129 and 198, Longmans, Greenand Co., London, England.

